1. Field of the Invention
This invention relates to a method of fabricating a photomask for use in personalizing a dielectric substrate and pertains, more particularly, to a photomask having at least three zones of light transmissibility therein. Such a photomask would be employed in permanently transferring artwork images, usually representative of electronic circuitry, to a metalized ceramic substrate.
2. Description of the Prior Art
The personalizing of a ceramic or any other dielectric substrate with the pads, lands and lines of electronic circuitry usually requires stratification of more than one metal to satisfy such diverse requirements as adequate adhesion, conductivity and corrosion resistance. Often the multi-metal strata must be complete in some areas and partial in others as dictated by design requirements. Photofabrication of such circuitry elements required several etching and/or deposition steps in varying locations on the single substrate surface.
This requirement led, in turn, to the need for multiple photoresist applications to mask off on location or pattern while exposing another. In attempting to reduce the number of photoprocess steps, which would result in obvious savings of time and money, attention was first focused on attempts to "double-process" a single resist layer (see International Business Machines Technical Disclosure Bulletin, Volume 14, No. 6, Page 1929, November 1971). Generally, this involved the exposure and development of certain areas using a positive resist, subsequent etching or plating, exposure and developing of other areas using the same photoresist and a second plating or etch step. This improvement only eliminated a stripping and reapplication step. The costly alignment and exposure steps still had to be repeated.